Hunter Syndrome (Mucopolysaccharidosis II, MPS II)

Abnormalities in an individual's genetic makeup cause genetic disease.

• Hunter syndrome is a genetic disease caused by a deficient or absent enzyme that is X-linked recessive. Hunter syndrome occurs mainly in males and produces symptoms starting at ages 2 to 4 years.
• Symptoms of Hunter syndrome range from mild to severe and may include the following:
  • Facial changes
  • Head enlargement
  • Abnormal bone size and shapes
  • Tongue protrusion
  • Voice changes
  • Joint stiffness
  • Small growths on the skin
  • Distended abdomen
  • Enlarged internal organs
  • Diarrhea
  • Developmental delays (physical and mental)
• The cause of Hunter syndrome is due to an abnormal X chromosome that males usually inherit from the mother.
• The two major risk factors for Hunter syndrome are a mother with an abnormal X chromosome, and her offspring is male.
• A large number of pediatric specialists may treat a child with Hunter syndrome because of the wide range of symptoms that can develop. If the child lives to adulthood, similar specialists may be involved in care for the adult with Hunter syndrome.
• Hunter syndrome is sometimes difficult to diagnose. Early symptoms include facial changes, and the definitive diagnosis depends on the genetic analysis of the child's X chromosome.
• Treatment for Hunter syndrome is mainly confined to reducing symptoms or slowing symptom development with idursulfase (Elaprase). There is no cure for Hunter syndrome.
• Complications of Hunter syndrome can involve all of the organ systems, including respiratory, skeletal, connective tissue, cardiac, and brain (neurologic).
• Life expectancy for individuals with Hunter syndrome ranges from about 10 to 20 years of age.
• There are two other terms that are sometimes confused with Hunter syndrome: 1) Bow Hunter Syndrome; and 2) Hurler Hunter syndrome (mucopolysaccharidosis I and MPS I), an inherited disease that produces similar symptoms and outcomes to Hunter syndrome.

Hunter syndrome is a very rare X - linked recessive inherited disease. Hunter syndrome is a lysosomal storage disease caused by deficient or absent enzyme, iduronate-2-sulfatase (I2S). This causes the accumulation of heparin sulfate and dermatan sulfate to accumulate in body tissues. Injury to various organs lead to the symptoms from the disease. Hunter syndrome is also termed Mucopolysaccharidosis II or MPS II.

The symptoms of Hunter syndrome usually are not present at birth, but begin in young children ages about 2-4, and may range from mild to severe. Symptoms include:

• Facial changes such as thickening of the lips, nostrils flaring and broadening of the nose
• Head enlargement (macrocephaly)
• Tongue protrusion
• Hoarseness and/or deepening of the voice
• Changes in the bones produces abnormal bone size and/or bone shape
• Joint stiffness
• Small whitish growths on the skin
• Enlarged internal organs
• Distended abdomen
• Diarrhea
• Delayed development (for example, child does not meet normal development such as talking or normal motor skills like walking)
• Behavior that becomes aggressive
• Reduced or stunted growth

The cause of Hunter syndrome is an abnormal gene X chromosome that is inherited from the mother. This genetic abnormality allows mucopolysaccharides to accumulate in large amounts in cells, the blood and in connective tissues. This accumulation causes damage to cells and organs. It is a progressive disease that usually causes a shortened lifespan and major developmental abnormalities. Hunter syndrome occurs almost exclusively in males.

The two major risk factors for Hunter syndrome are a family history of Hunter syndrome, and being male, since predominantly males are affected. Females rarely get the disease because they inherit two X chromosomes so that even if one is defective, the other X chromosome with functioning genes usually can compensate for the abnormality.

There are a number of physician specialties that may treat Hunter syndrome. Treatment is geared toward managing the symptoms and complications of the disease. There is no cure for this progressive disease. Specialties of physicians that may be involved in caring for persons with Hunter syndrome include pediatric pulmonologists, critical-care specialists, emergency medicine specialists, orthopedists, geneticists, rheumatologists, internal medicine specialists, neurologists, surgeons and child psychologists. If the child survives to adulthood, similar specialists that treat adults may be consulted.

The diagnosis of Hunter syndrome is sometimes difficult because of its slow onset and overlapping symptoms with other childhood problems. Early signs that suggest a diagnosis include facial changes in the child. The pediatrician then usually completes a detailed personal family history and physical examination of the individual. Blood, urine, and/or tissue samples can be tested for either the deficient enzyme or excess mucopolysaccharides. Definitive diagnosis is done by a genetic analysis of the child's X chromosome(s). There is no routine newborn screening available to diagnose Hunter syndrome. However, in families with known Hunter syndrome, prenatal testing of the amniotic fluid or placental tissue may be available and can verify the diagnosis in a fetus.

Currently, treatment is geared toward managing symptoms and complications as there is no cure for this progressive disease. Treatments depend on the organ(s) involved and can include

• assistance with breathing,
• physical therapy to maintain functions,
• drainage of fluids to build up in the brain and/or spinal cord,
• heart valve replacement, and
• other treatments such as behavior management.

Some physicians utilize specific enzyme therapy to replace a child's deficient enzymes. The treatment is termed enzyme replacement therapy (ERT) using an IV–infused enzyme (Elaprase). It is still being refined along with gene therapy to replace the abnormal part of the X chromosome.

Complications of Hunter syndrome are:

• Respiratory complications (thickened tongue can obstruct breathing)
• Skeletal problems (abnormal bone development and joint problems)
• Connective tissue (accumulation of mucopolysaccharides can cause swelling)
• Heart (thickened tissue resulting in vessel narrowing)
• Brain (hydrocephalus)
• Nervous system (thickened membranes impede nerve functions)
• Reduced ability to recover from most illnesses

The life expectancy the person with Hunter syndrome is reduced and ranges from about 10 to 20 years of age. However, with mild disease, some individuals live into adulthood. Obstruction of breathing or heart disease are the major causes of death.

Hunter syndrome is also known as

• Mucopolysaccharidosis II,
• MPS II, and
• MPS Disorder II.

Some researchers use subtypes such as MPS IIA and MPS IIB (MPS IIA is severe disease while MPS IIB is mild disease).

There are two other terms that are sometimes confused with Hunter syndrome.

1. The first is Bow Hunter Syndrome, a problem when a person rotates their head and causes circulation ischemia in the dominant vertebral artery that produces attacks of vertigo, nystagmus and ataxia (inability to walk normally).
2. The second term is Hurler Hunter syndrome. This term also is known as Hurler syndrome (mucopolysaccharidosis I and MPS I), and is closely related to Hunter syndrome because it also is an inherited disease that results in a lack of an enzyme called alpha- L-iduronidase, which produces similar symptoms and outcomes to Hunter syndrome.